Hmm. I've been thinking about this one and I think you're wrong. I think it should be either "never ever assume" or "never, ever, assume". The first I think is probably more accurate to modern usage as a colloquialism; it's probably very close to being a hyphenated word. The second makes more grammatical sense as "ever" is being used to parenthetically refine "never" (and specifically isn't a second adverb for "assume", since "never" and "ever" are antonyms), but I'm having trouble coming up with a parallel using different words, which is why I'm leaning towards colloquialism.